1. Field of the Invention
The present invention relates to a feeding method and a feeding device with high-frequency excitation for use in a plasma apparatus.
2. Description of the Related Art
In a general plasma apparatus, an electrode is disposed in a reaction chamber and a high-frequency power supply is connected to the electrode through a matching box. It is intended that the matching box matches the power supply impedance of the high-frequency power supply to the impedance of a plasma produced in the reaction chamber, the matching box being composed of lumped parameter circuit elements.
The matching box and the electrode in the reaction chamber are connected through a feeding cable. Where the electrical length of the feeding cable is short enough to be ignored, for example, when it is shorter than approximately 25 cm, the electrode can be substantially directly connected to the output side of the matching box and it is still possible to effectively transmit the electric power. However, in reality, it is difficult to mount the matching box near the electrode due to the inside and outside structure of the reaction chamber, and in such a case, the electrical length of the feeding cable is not short enough to be ignored. Therefore, when the matching box cannot be mounted near the electrode, standing waves are caused by reflections in the feeding cable, the feeding cable has an increased electric power loss, and the transmitting efficiency of the electric power is lowered.
Conventionally, in order to decrease the standing wave ratio in the feeding cable, as shown in FIG. 11, a plurality of coaxial cables 3 used as feeding cables are arranged in parallel so as to feed the electric power from a matching box 4 to an electrode 2 in a plasma chamber 1.
Assuming that the inductance and the capacitance of the coaxial cable per unit length are respectively L.sub.0 and C.sub.0, the characteristic impedance Zc of the cable is represented by the following equation: EQU Zc=(L.sub.0 /C.sub.0).sup.1/2 [ 1]
If N such cables are connected in parallel, the inductance is L.sub.0 /N, the capacitance is NC.sub.0, and the characteristic impedance Zc.sub.1 is: EQU Zc.sub.1 .apprxeq.{(L.sub.0 /N)/NC.sub.0 }.sup.1/2 =Zc/N [2]
For example, if two coaxial cables having the characteristic impedance of 50.OMEGA. are connected in parallel, the characteristic impedance is 25.OMEGA.. Therefore, by connecting a plurality of coaxial cables in parallel, it is possible to bring the characteristic impedance of the feeding cable close to the value of the plasma impedance.
However, since the plasma impedance is a capacitive load, it is difficult to achieve a perfect match the above method, and there is a problem that standing waves in the feeding cable lower the transmitting efficiency of the electric power.